See Figure 1Z20, Z22, Z24 and Z24i Engines
These engines have two spark plugs per cylinder. This arrangement allows the engine to burn large amounts of recirculated exhaust gases without affecting performance. In fact, the system works so well it improves gas mileage under most circumstances.
Both spark plugs fire simultaneously, which substantially shortens the time required to burn the air/fuel mixture when exhaust gases (EGR) are not being recirculated. When gases are being recirculated, the dual spark plug system brings the ignition level up to that of a single plug system which is not recirculating exhaust gases.